The prevalence of musculoskeletal +___________ impairments of the spine in the U.S. is estimated to be greater than 18 million-over one-half of all musculoskeletal impairments. Biomechanical testing of osteoligamentous spinal specimens has become fundamental to the investigation of the role of mechanical loads (i.e., forces and moments) in the etiology and management of spinal impairments. There is no consensus of opinion how loads are to be applied to a specimen to best represent in-vivo loading conditions. Current arguments center on whether biomechanical tests of spine are best done in a "load control" mode or a "displacement control" mode. However, evidence suggests that in-vivo, the spine does not operate exclusively in either of these control modes, but rather in a combination of load and displacement control-i.e., "hybrid control.' Existing spine testing systems fundamentally do not have the capacity to operate in hybrid control, therefore posing an obstacle to realistic control/application of experimental loads. The leading edge of hybrid control development and implementation is in robotics-thus the objective of this proposal is to develop a robotics-based spine testing system, and use it to study in-vitro spinal biomechanics under hybrid control. Four specific aims are proposed: (1) Develop and validate a robotics-based spine testing system with programmable hybrid control. (2) Delineate the in-vitro kinetics (i.e., load-displacement response) of human lumbar functional spinal units (FSUs) under hybrid control. (3) Determine the effects of degeneration and (4) surgery on in-situ forces of the disc, ligaments and facet joints. A computer-based robotic controller will be developed and validated using experimental and analytical approaches to enable an existing commercial robot to apply loads to human lumbar FSUs under hybrid control. Overall kinetic response as well as functional role of the structural elements of the specimens will be delineated during external loading of the FSU. Radiographic and morphological analyses will be done to correlate biomechanical properties with degenerative changes. The knowledge gained from this study will improve our understanding of the biomechanics of the spine under control modes more representative of in vivo conditions, and will help to elucidate the role of degenerative changes in the load-bearing of the spine.